Behavior of serum albumins of different species was compared with regard to electrophoretic mobility under native conditions, molecular mass and isoelectric point in presence or absence of denaturing agents. In addition to serum samples from human, horse, cow, pig, cat, dog, sheep, goat, chicken, rabbit, mouse, and rat, commercially available standards or albumin preparations obtained by chromatographic procedures were analyzed. Feline and canine albumins showed marked differences in all investigated methods. Our experiments also revealed several factors influencing albumin patterns, especially during isoelectric focusing in immobilized pH gradients or in the presence of carrier ampholytes.